CHAPTER VI 



MOVEMENT OF MATERIALS IN THE PLANT 



§i. General Occurrence of Movement of Materials. — From previous 

 statements it is clear that the essential materials are not always directly ab- 

 sorbed by the plant organs in which they are ultimately used. Organic materials 

 are produced from inorganic substances in the green leaf, but the leaf itself can 

 absorb only carbon dioxide. The other materials (water and mineral constitu- 

 ents) that are necessary in the formation of organic compounds are absorbed 

 by the roots, and usually travel long distances before finally reaching the leaves. 

 Similarly, organic materials are frequently used in large quantities in organs 

 where they are not produced; for instance, in all growing parts that lack chloro- 

 phyll. This is especially true of organic materials that are elaborated from 

 inorganic compounds; new kinds of organic substances may of course be pro- 

 duced in any region of the plant, from other organic substances that have been 

 previously formed there, or that come from elsewhere. The organic substances 

 that are requisite for the formation of new cells come to these cells from 

 the leaves, and they also frequently travel long distances before reaching the 

 point where they are used, as in the case of growing root-tips. It is clear, 

 therefore, that there is a general movement of materials within the plant. 



The compounds occurring in plants may be in the solid as well as in the 

 liquid or gaseous condition. Solid substances, however, must first pass into 

 solution before translocation can occur, since otherwise they cannot pass 

 through cell walls. The study of the movement of materials in plants may, 

 accordingly, be reduced to a consideration of the movement of gases and 

 of water and dissolved substances. 



§2. Movement of Gases. — Many air passages (intercellular spaces) are 

 always present in the cortex of stems and roots as well as in the parenchymatous 

 tissues of leaves. The lenticels, small openings in the bark, and the stomata 

 also, bring these passages into direct connection with the external air, and the 

 internal atmosphere is thus always under the same pressure as that of the air 

 outside, while renewal of the internal air may readily occur through openings 

 to the outside. 



Gas exchange through the cortex of water plants is greatly hastened by 

 differential diffusion of air, which was first observed in the leaves oiNelumbium 

 speciosum. 1 The leaf of this plant consists of a round leaf-blade, from the 

 center of the lower surface of which the petiole projects. Stomata occur only 



1 Barthelemy, A., De la respiration et de la circulation des gaz dans les vegetaux. Ann. sci. nat. Bot. V . 

 19: 131-175. 1874. [See also, for observations and a better explanation: Ohno, N., Ueber lebhafte 

 Gasausscheidung aus den Blattern von Nelumbo nucifera. Zeitschr. Bot. 2: 641-664. 1010. [Rev. 

 by Livingston in: Plant world 14: 7-2-73- ion) 



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